uprobes: document the usage of mm->mmap_lock

The comment above uprobe_write_opcode() is wrong, unapply_uprobe() calls
it under mmap_read_lock() and this is correct.

And it is completely unclear why register_for_each_vma() takes mmap_lock
for writing, add a comment to explain that mmap_write_lock() is needed to
avoid the following race:

	- A task T hits the bp installed by uprobe and calls
	  find_active_uprobe()

	- uprobe_unregister() removes this uprobe/bp

	- T calls find_uprobe() which returns NULL

	- another uprobe_register() installs the bp at the same address

	- T calls is_trap_at_addr() which returns true

	- T returns to handle_swbp() and gets SIGTRAP.

Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20240801132709.GA8780@redhat.com